1. Field of the Invention
This invention relates to the field of human-powered vehicles. More specifically, the invention comprises a transmission for converting a reciprocating lever motion into rotary motion.
2. Description of the Related Art
Pedal-powered vehicles have been in common use for over a century. Bicycles are of course the most common, but tricycles and even quadracycles are also well known. The present invention is applicable to any pedal-powered vehicle, as well as any other machine in which a moving lever is used as a power input (including machines in which human arm power is used). The illustrations show the invention's use with a bicycle or tricycle frame, but this should not be viewed as limiting.
FIG. 1 illustrates a prior art pedal mechanism for a bicycle. Most bicycles have a conventional frame including seat tube 14, down tube 16, bottom bracket 12, right chain stay 20, and left chain stay 18. Bottom bracket 12 is a cylindrical housing which is the attachment point for crank mechanism 10. The frame components—including the bottom bracket—are typically welded together.
Left crank 22 and right crank 26 extend outward from a rotational assembly mounted within bottom bracket 12. Left pedal 24 is attached to left crank 22, while right pedal 28 is attached to right crank 26. The two cranks are mounted 180 degrees apart, so that the driving force provided by each of the user's legs is 180 degrees out of phase with the opposing leg.
One or more chain rings 30 are attached to the crank mechanism. These rotate with the pedals. A chain is engaged with one of these chain rings and the chain transmits linear force to the rear drive sprocket. A front selector mechanism moves the chain between the different chain rings 30. A rear selector mechanism moves the chain between the different rear drive sprockets. In combination, the two selector mechanisms determine an overall drive ratio between the pedals and the rear wheel. These features allow a wide range of drive ratios.
The prior art pedal motion is purely rotational. In most instances power is provided only during the “down stroke”—when a pedal is descending through the forward portion of its arc. However, some riders use toe clips to actually attach the shoes to the pedals so that a rider may pull on the pedal during the upward stroke. A strap over the top of the shoe may also be provided for this purpose.
Other prior art designs have used reciprocating pedal motions instead of purely rotational motion. In this approach the pedals travel through an abbreviated arc. The pedals preferably reciprocate so that as one pedal is being pushed down when the other is rising up. In the prior art the user applies force by pushing on the pedal during the down stroke. The opposing pedal is rising to the top of its arc but it is not providing any force input.
Reciprocating pedal designs are comfortable for most users, since the human leg is well adapted to provide a pushing stroke. They work particularly well in recumbent vehicles, where the user has a back rest to push against. However, the prior art reciprocating designs have several drawbacks. First, the return pedal motion (the upward travel in the arc) has previously been provided by a return spring. This spring must be elongated during the down stroke—meaning that the user is expending some effort in simply stretching the spring. Most of this energy is returned when the spring contracts on the upward stroke, but the irreversibilities reduce the overall efficiency.
Second, the prior art reciprocating designs have constrained the arc through which each pedal could travel. It would be better to provide a variable arc length since this would accommodate differing leg lengths, differing abilities, and differing preferences. The present invention provides a reciprocating treadle design which excludes the return spring and allows operation over a variable arc length.
Finally, the prior art designs only allow the user to apply force in the push stroke. The present invention allows force to be applied to both pedals simultaneously—pushing on one and pulling on the other. These differences and advantages will be explained in the following descriptions.